With the recent development of information communication technology, a variety of wireless communication techniques are being developed. From among them, a Wireless Local Area Network (WLAN) is a technique for wirelessly accessing the Internet at homes or companies or in specific service providing areas by using portable terminals, such as a Personal Digital Assistant (PDA), a laptop computer, and a Portable Multimedia Player (PMP), based on wireless frequency technology.
A lot of standardization tasks are being performed since Institute of Electrical and Electronics Engineering (IEEE) 802 (i.e., the standardization organization of WLAN technology) was established on February, 1980. WLAN technology initially supported a speed of 1 to 2 Mbps through frequency hopping, band spreading, and infrared communication by using a frequency of 2.4 GHz according to IEEE 802.11, but recently may support a maximum speed of 54 Mbps by using Orthogonal Frequency Division Multiplexing (OFDM). In addition, in IEEE 802.11, standardizations for various techniques, such as the improvement of Quality of Service (QoS), Access Point (AP) protocol compatibility, security enhancement, radio resource measurement, wireless access vehicular environments, fast roaming, a mesh network, interworking with an external network, and wireless network management, are being put to practical use or developed. Furthermore, in order to overcome a limit to the communication speed that was considered as being weakness in the WLAN, IEEE 802.11n has recently been established as a technical standard. An object of IEEE 802.11n is to increase the speed and reliability of a network and to extend the coverage of a wireless network. More particularly, in order to support a High Throughput (HT) having a maximum data processing speed of 540 Mbps or higher, minimize an error in transmission, and optimize the data speed, IEEE 802.11n is based on Multiple Inputs and Multiple Outputs (MIMO) technology using multiple antennas on both sides of a transmitter and a receiver. Furthermore, this standard may use not only a coding scheme for transmitting several redundant copies in order to increase data reliability, but also Orthogonal Frequency Division Multiplex (OFDM) in order to increase the speed.
With the widespread use of the WLAN and the diversification of applications using the WLAN, there is a recent demand for a new WLAN system to support a higher throughput than a data processing rate supported by the IEEE 802.11n. However, an IEEE 802.11n medium access control (MAC)/physical layer (PHY) protocol is not effective to provide a throughput of 1 Gbps or higher. This is because the IEEE 802.11n MAC/PHY protocol is designed for an operation of a single station (STA), that is, an STA having one network interface card (NIC), and thus when a frame throughput is increased while conforming to the conventional IEEE 802.11n MAC/PHY protocol, a resultant additional overhead is also increased. Consequently, there is a limitation in increasing a throughput of a wireless communication network while conforming to the conventional IEEE 802.11n MAC/PHY protocol, that is, a single STA architecture.
Therefore, to achieve a data processing rate of 1 Gbps or higher in the wireless communication system, a new system different from the conventional IEEE 802.11n MAC/PHY protocol (i.e., the single STA architecture) is required. A very high throughput (VHT) WLAN system is a next version of the IEEE 802.11n WLAN system, and is one of IEEE 802.11 WLAN systems which have recently been proposed to support a data processing rate of 1 Gbps or higher in a MAC service access point (SAP).
The VHT WLAN system allows simultaneous channel access of a plurality of VHT STAs for the effective use of a radio channel. For this, multi-user multiple input multiple output (MU-MIMO)-based transmission using multiple antennas is supported. A VHT access point (AP) can perform spatial division multiple access (SDAM) transmission which concurrently transmits spatial-multiplexed data to a plurality of VHT STAs. When data is concurrently transmitted by distributing a plurality of spatial streams to the plurality of STAs through a plurality of antennas, an overall throughput of the WLAN system can be increased.
Meanwhile, the IEEE 802.11e standard supports a direct link setup (DLS) service for directly transmitting data between STAs without the aid of an AP. The DLS service sets up a direct link (DL) between a DLS initiating STA (i.e., a DLS initiator) and a DLS responding STA (i.e., a DLS responder), and thereafter directly transmits/receives a data frame through the DL. Details of the DLS service may be found in the section 7.4.3 and the section 10.3.25 of IEEE Standard for Information technology—Telecommunications and information exchange between systems—Local and metropolitan area networks—Specific requirements, Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications' introduced in June, 2007.
In order to more effectively use a radio resource in a WLAN system supporting MU-MIMO, it is possible to consider a method of simultaneously performing data by using a DLS service between STAs which are not targets of MU-MIMO transmission while MU-MIMO transmission is achieved between an AP and a plurality of STAs. In this case, MU-MIMO transmission of the AP may act as interference to an STA which receives data by using the DLS service. This causes decrease in data transmission reliability, and impairs effective utilization of a radio resource, thereby decreasing an overall throughput of the WLAN system. Accordingly, there is a need to consider a method of avoiding mutual interference when MU-MIMO transmission and data transmission using the DLS service are simultaneously performed.